I have to agree with everyone else. A tshirt is a great end of year totem. And an after-show hotdog and pop is good for a nightly award too (You never realize how good byproducts are til you destroy for 4 hours haha).
I've always thought that taking a group photo after one of the performances, then getting copies made would be something nice to present to all involved. Maybe use photoshop and put the haunts name/year/character names etc.
Another idea...you could go to the dollar store and get those cheap mini tombstones and use a mini foam cutter to carve things like, "Best use of (fill in the blank)" "Best make-up" etc, and give them out at a post-party.
Jaks is dead on about the banquet idea too, nothing beats a good celebration of a job well done. Especially when you have great "remember when this happened.." tales to share!
In the Shadows...We are Legion